The cytotoxicity of malignant pleural effusion lymphocytes (MPEL) against autologous tumor cells (ATC) were compared with that of peripheral blood lymphocytes (PBL). It was demonstrated that the cytotoxicity of PBL was higher than that of MPEL (P < 0.001), but the cytotoxicity and expansion of MPEL-activated by rIL-2 was much higher than that of PBL activated by rIL-2 (LAK cells) (P < 0.001). This shows that local immune reaction of the pleural cavity of patients with malignant pleural effusion was in the state of suppression. MPEL activated are better effector cells than LAK cells in tumor adoptive immunotherapy.